Realistically, what will happen if mining power moves to an alternative chain but Bitcoins are still what is traded at exchanges, is that miners will move back very quickly. The first one to move will gain the most economically, so game theory dictates the move back will be swift.

In the end, miners mine what people buy. It's that simple.

Any speculation as to what happens if all miners move as one, motivated by ideology instead of economics, is just that.